In the case of conventional power transistors, for example, the breakdown voltage Vbr(V) and the forward resistance Ron★A(Ωcm2) of the drift path depend on the one hand on the doping concentration of foreign substances in the drift path, and on the other hand on the length of the drift path, where Ron is the value of the resistance formed by the drift path, and A is the cross-sectional area of the resistance formed by the drift path. The expression drift path means that region in the semiconductor body of the semiconductor component across which the major proportion of the breakdown voltage is dropped.
A high doping concentration in the drift path and its short configuration lead to a low forward resistance, and to a low permissible reverse voltage. Conversely, a low doping concentration in the drift path and its long extent result in a high permissible reverse voltage and a high forward resistance. Against this background, the problem is to achieve a high permissible reverse voltage and a low forward resistance for high blocking capability semiconductor elements having a drift path.
This problem is partially solved by so-called compensation components, such as CoolMOS transistors, in which additional p-doped regions are arranged in the drift path such that, when in the forward-biased state, they do not significantly impede the current flow between the source electrode and the drain electrode, but largely compensate for the charge in the drift path when the space charge zone is in the reverse-biased state. The introduction of these additional p-doped regions with the opposite charge polarity to the drift path prevents the charges in the drift path from producing an excessively high electrical field. One disadvantage of this solution is that it is complex to produce such p-doped regions in an otherwise n-doped drift path.
DE 198 40 032 C1 discloses a modification of this compensation component, in which the doping in the drift path and/or in the compensation regions can be set such that, when reverse-biased, the electrical field has a profile which rises from both sides of the drift path. The maximum electrical field strength is thus not reached in the vicinity of one end of the drift path, but in its interior, for example approximately at the center. In the case of n-conductive components, for example, in the source-end area of the drift path, the p-doped compensation regions may be more highly doped for this purpose, while those in the drain-end area of the drift path may be more lightly doped than the n-doped drift path. This can be achieved not only by vertical variation of the doping in the compensation regions, but also by vertical variation of the doping in the drift path. Components such as these are distinguished by high avalanche resistance as well as a current load capacity in breakdown, and by increased tolerance to manufacturing fluctuations. Nevertheless, the manufacturing effort remains considerable.
The document WO 02/067332A2 discloses a trench structure which is introduced into the drift path and whose walls have an insulating layer, with the trench structure being filled with a semi-insulating material, preferably polysilicon. This design makes it possible to improve the electrical field distribution in the drift path so as to allow heavier doping in the drift path without excessive field strength peaks occurring in the depth of the reverse-biased pn junction, which would reduce the blocking voltages. Oxides, preferably silicon oxides, are arranged on the trench walls as the dielectric insulation material. One disadvantage is that, when reverse-biased, a considerable leakage current can flow via the semi-insulating layer arranged in the drift path. This becomes greater the more effectively the semi-insulating layers in the trench structure influence the profile of the electrical field in the drift path.
Finally, US 2003/0047768 discloses the profile of the electrical field in the drift path being influenced with the aid of field plates, which are introduced into a trench structure, in such a way that heavier doping can be provided in the drift path. A field plate such as this can be arranged in a trench structure, such as it is isolated from its semiconductor body, in the case of vertical semiconductor components. However, the amount of effort for production of a semiconductor component such as this is relatively high. In addition, the required thickness of isolation layers between the semiconductor body of the semiconductor component and the field electrodes increases in proportion to the desired breakdown voltage. For this reason, this trench structure is sensibly applicable in practice in semiconductor components only up to maximum blocking voltages of 200 V.